Whenever you need to find the sum of an arithmetic series use this formula:
\[ \frac n 2 \left( 2a+ (n-1) \right)\times d) \tag{1}\]
a= first term
n= number of terms
d= common difference
Here, \(n= 38, a = 137 \) and \( d = -12\).
Substituting these in \((1)\), and assuming my algebra is right you answer should be \(-3230 \)